The mechanism by which parathyroid and calcitonin cells sense and respond to small changes in extracellular calcium concentrations is unknown. We have demonstrated that the secretory stimulus of hypocalcemia induces dramatic hyperpolarization of the parathyroid cell membrane, a response which could be an important coupling signal in the sequence of events leading to secretion. The goal of this proposal is to characterize the ionic mechanism and calcium specificity of the parathyroid hyperpolarization and to investigate the sensitivity of the calcitonin cell membrane potential to calcium.